Taming the Sun: The challenges of Facade Solar Control

24Jul

Taming the Sun: The challenges of Facade Solar Control

Sun is one of the most important natural resources that a building can harvest, but to do so efficiently, a sun control strategy along with specific shading devices must be applied, to have the desired result on the amount of sunlight and heat entering a building. For example, in warm, sunny climates we need to minimize the solar gains, as it results in excessive cooling loads. In cold and temperate climates, incident solar rays on the south oriented envelope contribute to active and passive solar heating. Additional to the above, control of the direct and diffuse daylight to eliminate glare is a major concern. In every building, sun control using the relevant shading devices is challenging and often results in different facade types per building orientation. In this post, we will present the basic principles of solar shading and the key idea before selecting the appropriate shading device for our needs.

Solar Geometry Basics

The design of effective shading devices will depend on the solar orientation of a particular building facade. To properly design shading devices, it is necessary to understand the position of the sun in the sky during the year. The position of the sun is expressed in terms of altitude and azimuth angles (along with many other angles too, but let’s keep in simple). The altitude angle is the angle of the sun above the horizon, achieving its maximum on a given day at solar noon - which slightly differs from 12pm o’ clock. The azimuth angle, also known as the bearing angle, is the angle of the sun's projection onto the ground plane relative to the south for the northern hemisphere and to the north for the southern hemisphere. For any given location, the position of the sun is well known either by using old-school sun path diagrams or online tools like SunEarthTools.

Designing efficient shading systems

Architectural shading devices, either fixed or movable, can have a dramatic impact on building aesthetics. This impact sometimes is good, but in most of the cases is destroying the building’s architectural design intent, especially when applied to existing structures. For new developments, even if it’s still a major challenge, the earlier in the facade design process that shading devices are considered, the more likely they are to be sleek and well-integrated in the overall building envelope. In terms of quantification measures, the degree of window shading is a major consideration. Both the Projection Factor (PF - The ratio of the distance the overhang projects from the window surface to its height above the sill of the window it shades) for exterior shading and the Shading Coefficient (SC - The ratio of solar heat gain through a specific glazing system to the total solar heat gain through a single layer of clear, double-strength glass) of glass must be evaluated.

Shading Strategies for Exterior Shading Devices

Exterior shading devices, either attached to the primary building skin or as a feature of the building skin itself, are typically more effective than interior devices and block solar heat gains more efficiently. Some key ideas for choosing the best shading devices are below:

If the architect or the owner does not accept attachments, the trick is to design the building with such geometry that shades itself (window set back in deeper wall section or extended non-bearing wall elements).

Horizontal shading devices, such as awnings, overhangs or recessed windows, are usually applied in the south orientation, but also have some impact on east and west facades. On the contrary, there is no function at all for the north oriented surfaces.

For overhangs, usually applied in windows, less projection can be achieved by dropping the edge or putting it on a small slope. When you need to shade large height span curtain wall, to achieve less projection you have to break up the overhang in narrower parts and position them all over the glazed surface height.

Dropped edge can also be substituted by small scale louvers to allow more direct daylight. If you need more diffuse light, while not reducing shading, there is the option to use small scale louvers instead of a solid surface.

Vertical shading devices are very convenient for west and east facades. They are also kind of useful in north orientation to block early morning and late afternoon low sun. However, morning sun does not contribute too much to heat gains, so if your budget constraints are tight, the primary goal is to facilitate shading devices in west and south shading only.

Glare reduction is also one very important issue to consider. The bright sky has to be partially blocked from the occupant’s view and that is often achieved by exterior shading only.

There is always an argument between fixed and movable exterior shading systems. Efficient-wise, fixed systems do an adequate job, but movable do an exceptional job, especially in lowering energy demands when also combined with sun sensors and are controlled by a central BMS. Of course, the higher installation and maintenance costs are a barrier that has to be considered, but in the last years, many reliable and cost-effective motorized systems have been available.

Shading Strategies for Window Plane Systems

It’s not only a battle between exterior and interior, as many systems applied on the window plane also has to be considered as an option in some cases. Some key ideas for choosing such devices are below:

Roller shades are a cheap movable alternative. They are not highly effective, but acceptable.

Many glass suppliers say that dark glazing is a must if you want to reduce heats gains. Glazing treatments, such as reflective coatings, tints, and films, can reduce heat transfer but allow direct sun penetration (with reduced density), which in regards to occupants’ perception is not very effective.

Fritted glass, on the contrary, could be a good acceptable option for controlling sunlight, but may also increase glare.

Devices like in between glass Venetian blinds, either fixed or movable, are also an option for efficient sun control, but limitations apply, most of them described in the next section.

Shading Strategies for Exterior Shading Devices

Interior shading alone has limited ability to control solar gains and are less effective than exterior ones because the sun has already entered the building. They also depend on occupants’ behavior, which can’t be patronized. Some of the strategies used for better results with interior shading are below:

If the only available shading is interior, light colored blinds or louvers must be used to reflect solar heat back out. Semi-translucent devices are also acceptable, but may not be able to control glare during the cooling period.

Interior shading devices are best used in addition to primary exterior shading, to provide user-oriented glare control and backup shading, according to individual needs.

Interior shading devices that totally block light must be avoided at any cost. That is also applied for dark colored interior systems too, such as total black out rollers or curtains.

Invest in a bespoke facade shading solution

There are numerous options and design possibilities in building envelopes that provide sun control and shading. For many architects and developers, conventional “off the shelf” solutions are not performing well and thus are not what is required. In these cases, bespoke development and engineering may be better suited to offer the desired results, either for a second building skin or new projects. Working with a specialist company with in-house design and engineering experts can deliver the ideal shading solution for your needs.